The present invention relates to a process for preparing a flame-retardant phosphorus-containing unsaturated polyester.
Flame-retardant unsaturated polyesters can be obtained by adding flame retardants such as aluminum hydroxide and halogen combined with antimonous oxide or phosphorus/halide to unsaturated polyester, followed by blending the resulting polyester with polymerizable monomer containing a vinyl group. This method has the advantages of convenience and low manufacturing cost. However, to attain the flame-retardant effects, the amount of the additives, i.e. flame retardants, to be added is large, generally 50 wt %. Also, the additives sometimes are detrimental to the physical properties of the polyester resin-blends, and cause the migration of the flame retardants to the surface of the fabricated products, resulting in the lowering of the flame-retardant effect. Moreover, increments of inorganic flame retardants causes the increase of viscosity of polyester, resulting in the difficulties in their processing, such as extrusion or injection molding.
Flame-retardant unsaturated polyester can also be obtained by directly reacting flame retardants, such as halogen or phosphorus/halides into the polyester molecular structure, followed by blending the resulting polyester with polymerizable monomer containing a vinyl group. The polyesters thus prepared are free of the disadvantages mentioned above. However, the manufacturing cost is high, and thus this method is also unsatisfactory. For example, in U.S. Pat. No. 3,662,025, it discloses a phosphorus-containing unsaturated polyester which is prepared by blending a hydroxyl terminated unsaturated polyester and an ethylenically unsaturated monomer having a vinyl group. The hydroxyl terminated unsaturated polyester is prepared by reacting an unsaturated dicarboxylic acid or acid anhydride and a molar excess of glycol. The hydroxyl terminated unsaturated polyester is then reacted with a phosphonate intermediate which is the reaction product of a phosphonate and an organic hydroxy acid or a lactone. The process for preparing the hydroxyl terminated unsaturated polyester involves a three-step synthesis and requires charging the reactants in several batches, and thus is not economical.